This invention relates to fuser members useful for heat-fixing a heat-softenable toner material to a substrate. More particularly, the invention relates to materials usable as a toner release layer in a fuser member.
Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield the final desired toner image.
When heat-softenable toners, comprising, e.g., thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, at least one of which (usually referred to as a fuser roll) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll (usually referred to as a pressure roll) serves to press the receiver sheet into contact with the fuser roll. In some other fusing methods, the configuration is varied and the xe2x80x9cfuser rollxe2x80x9d or xe2x80x9cpressure rollxe2x80x9d takes the form of a flat plate or belt. The description herein, while generally directed to a generally cylindrical fuser roll in combination with a generally cylindrical pressure roll, is not limited to fusing systems having members with those configurations. For that reason, the term xe2x80x9cfuser memberxe2x80x9d is generally used herein in place of xe2x80x9cfuser rollxe2x80x9d and the term xe2x80x9cpressure memberxe2x80x9d in place of xe2x80x9cpressure rollxe2x80x9d.
The fuser member usually comprises a rigid core covered with a resilient material, which will be referred to herein as a xe2x80x9cbase cushion layer.xe2x80x9d The resilient base cushion layer and the amount of pressure exerted by the pressure member serve to establish the area of contact of the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip of the fuser member and pressure members. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness (often referred to as xe2x80x9cstorage modulusxe2x80x9d) and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
In some previous fusing systems, it has been advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roll and a fuser roll, by slightly modifying the shape of the pressure roll. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established, for example, by continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roll a sort of xe2x80x9cbow tiexe2x80x9d or xe2x80x9chourglassxe2x80x9d shape. This will cause the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roll begins to permanently deform to conform to the shape of the pressure roll and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation (alternatively referred to as xe2x80x9ccreepxe2x80x9d) of the base cushion layer of the fuser member is the greatest contributor to this problem.
Particulate inorganic fillers have been added to base cushion layers to improve mechanical strength and thermal conductivity. High thermal conductivity is advantageous when the fuser member is heated by an internal heater, so that the heat can be efficiently and quickly transmitted toward the outer surface of the fuser member and toward the toner on the receiver sheet it is intended to contact and fuse. High thermal conductivity is not so important when the roll is intended to be heated by an external heat source.
Optimal metal-particle filled elastomer fuser members have long been sought. At one time, it was predicted that:
xe2x80x9cThe metal of the metal-containing filler dispersed in the elastomer may be easily selected by one skilled in the art without undue experimentation by testing the metal-containing filler, such as a metal, metal alloy, metal oxide, metal salt or other metal compound, in an elastomer. The general classes of metals which are applicable to the present invention include those metals of Groups 1b, 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6b, 7b, 8 and the rare earth elements of the Periodic Table.xe2x80x9d (U.S. Pat. No. 4,264,181 to Lentz et al, column 10, lines 42-53; also U.S. Pat. No. 4,272,179 to Seanor, column 10, lines 45-54.)
This prediction of easy selection of the metal for a metal-containing filler has proven false in the face of later efforts in the art. A metal-containing filler which provides good results in one elastomer may provide very poor results in another elastomer, even if the elastomers are very similar.
U.S. Pat. No. 4,515,884 to Field et al, discloses a fuser member which utilizes metal oxide filled polydimethylsiloxane. The metal oxides are iron oxide and tabular alumina. Calcined alumina is described as being unsuitable per se. (column 9. line 50-column 10 line 47)
In U.S. Pat. No. 4,264,181 to Lentz et al, good results were obtained when lead oxide was used as a filler in various fluorocarbon elastomers (Viton E430(trademark), Viton E60C(trademark), Viton GH(trademark); Examples X, XI, XII). U.S. Pat. No. 5,017,432 to Eddy et al, on the other hand, teaches against the use of lead oxide in similar fluorocarbon elastomers (for example, Viton GF(trademark)) on the basis that it would produce an unacceptable fuser member. In these fluoroelastomers, cupric oxide is preferred.
U.S. Pat. No. 4,272,179 to Seanor and U.S. Pat. Nos. 4,264,181 and 4,257,699 to Lentz teach the use, as a release oil, of a polydimethylsiloxane that incorporates mercapto functional groups. These patents indicate that lead oxide filler in the outer elastomer layer interacts with the mercapto functionalized PDMS fluid to yield a release film on the surface of the fuser member.
Preparation of metal containing elastomers remains problematic. U.S. Pat. No. 4,515,884 to Field et al, and U.S. Pat. No. 5,017,432 to Eddy et al, cite large numbers of critical features or important aspects of their metal containing elastomers: choice of material (Field, column 9, lines 50-65 and column 10, lines 24-25), interaction of filler surface and elastomer (Field, column 9, lines 32-65), particle size (Field, column 10, lines 1-8 and lines 25-30; Eddy, column 9, line 65-column 10, line 3), concentration of metal-filler (Field, column 10, lines 9-23 and lines 31-47), capability of interacting with functional groups of release agent (Eddy, column 9, lines 26-30), reactivity of the metal filler with the elastomer (Eddy, column 9, lines 33-43), and acid-base characteristics of the metal filler (Eddy, column 9, lines 43-56). The lists of critical features and important aspects in Field and Eddy do not fully correlate. It is unknown whether this difference represents real differences in material characteristics or only differences in techniques and analysis.
In electrophotographic fuser systems, fuser members are commonly made with an overcoat layer of polysiloxane elastomer, polyfluorocarbon resin, or polyfluorocarbon elastomer.
Polysiloxane elastomers have relatively high surface energy and relatively low mechanical strength, but are adequately flexible and elastic and can produce high quality fused images. After a period of use, however, the self release property of the roller degrades and offset begins to occur. Application of a polysiloxane fluid during roller use enhances the ability of the roller to release toner, but shortens roller life due to oil absorption. Oiled portions tend to swell and wear and degrade faster.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rolls is condensation-crosslinked siloxane elastomer. Disclosure of filled condensation-cured poly(dimethylsiloxane) xe2x80x9cPDMSxe2x80x9d elastomers for fuser rolls can be found, for example, in U.S. Pat. Nos. 4,373,239; 4,430,406; and 4,518,655. U.S. Pat. No. 4,970,098 to Ayala-Esquillin et al teaches a condensation cross-linked diphenylsiloxane-dimethylsiloxane elastomer having 40 to 55 weight percent zinc oxide, 5 to 10 weight percent graphite, and 1 to 5 weight percent ceric dioxide.
A widely used siloxane elastomer is a condensation-crosslinked PDMS elastomer, which contains about 32-37 volume percent aluminum oxide filler and about 2-6 volume percent iron oxide filler, and is sold under the trade name, EC4952, by the Emerson Cummings Co., U.S.A. It has been found that fuser rolls containing EC4952 cushion layers exhibit serious stability problems over time of use, i.e., significant degradation, creep, and changes in hardness, that greatly reduce their useful life. MER test results correlate with and thus accurately predict the instability exhibited during actual use. Nevertheless, materials such as EC4952 initially provide very suitable resilience, hardness, and thermal conductivity for fuser roll cushion layers.
Some filled condensation-crosslinked PDMS elastomers are disclosed in U.S. Pat. No. 5,269,740 (copper oxide filler), U.S. Pat. No. 5,292,606 (zinc oxide filler), U.S. Pat. No. 5,292,562 (chromium oxide filler), U.S. Pat. No. 5,480,724 (tin oxide filler), U.S. Pat. No. 5,336,539 (nickel oxide filler). These materials all show much less change in hardness and creep than EC4952 or the PDMS elastomer with aluminum oxide filler. U.S. Pat. No. 5,292,606 and U.S. Pat. No. 5,480,724 disclose that tin oxide filler and zinc oxide filler can provide very good results in PDMS. Fluorocarbon resins like polytetrafluoroethylene (PTFE) or a copolymer of PTFE and perfluoroalkylvinylether, or fluorinated ethylenepropylene have excellent release characteristics due to very low surface energies, high temperature resistance, and excellent chemical resistance. Fluorocarbon resins are, however, less flexible and elastic than polysiloxane elastomers.
Polyfluorocarbon elastomers, have been extensively studied for the field of electrophotography. Continuously, the bane of such materials are their poor contamination. The tendency for fluoroelastomers to contaminate with toner is affected significantly by the ingredients needed to cure the fluoroelastomers. The worst of these being the metal oxide and hydroxides bases needed as dehydrofluorinating agents and as acid scavengers. Another difficulty with such systems is that the presence of the materials as curatives can also adversely affect coating quality an thus reduce the image gloss the fuser member is capable of imparting. Recently effort has been placed on using organic bases for these purposes. However, the choice of materials that can both perform the necessary functions above and withstand the cure temperatures are limited. There is a need for a fluoroelastomer cure system, which does not significantly impair toner release or lead to high contamination.
The present invention provides a fuser member with an overcoat layer that includes a polyfluorocarbon elastomer which is cured in such a way as to have minimal effect on the release properties or the surface properties of the material. At the same time, the fluoroelastomer is cured to a degree sufficient to impart the necessary mechanical integrity.
The present invention also provides an improved mixture of materials for forming a toner release layer.
This is achieved in a fuser member comprising a core and a layer overlying the core, the layer including a cured fluorocarbon random copolymer having subunits of:
xe2x80x94(CH2 CF2)xxe2x80x94,xe2x80x94(CF2CF(CF3)yxe2x80x94, or xe2x80x94(CF2 CF2)zxe2x80x94,
wherein x is from 30 to 90 mole percent,
y is from 10 to 70 mole percent,
z is from 0 to 34 mole percent; and
x+y+z equals 100 mole percent;
the layer further including particulate filler having zinc oxide and yellow iron oxide.
The layer incorporates particulate filler. The filler includes zinc oxide and yellow iron oxide. The zinc oxide is present in an amount from 5 to 30 parts based on 100 parts of fluorocarbon random copolymer. The yellow iron oxide is present in an amount from 5 to 30 parts based on 100 parts of fluorocarbon random copolymer. The composition may further include other fillers to impart such properties as thermal conductivity or cosmetic purposes. A brief list of such fillers includes but is not limited to tin oxide, copper oxide, graphite, carbon black, and aluminum oxide.
The fuser member of the invention, formed with a toner release layer that includes a metal oxide filled polyfluorocarbon elastomer, has a moderate surface energy.
Further, images formed using the fuser member the present invention attain a higher image gloss.
A further advantage of the present invention is that the need for strong bases to cure the fluoroelastomer is eliminated.
Yet another advantage of the present invention is that less offset occurs.